Heretofore, various lead-free piezoelectric compositions have been studied, including, for example, (Bi0.5K0.5)TiO3 (hereinafter, also referred to as BKT) and BKT-BiFeO3 (hereinafter, also BiFeO3 referred to as BFO) binary lead-free piezoelectric compositions. Their piezoelectric constants, however, are still small compared with those of lead-based piezoelectric compositions under the present circumstances (see, for example, PTL 1 and PTL 2 and NPL 1 and NPL 2). In addition, a solid solution of BKT and Bi(Fe,Co)O3 (hereinafter, also referred to as BFCO) which is derived from BiFeO3 by the replacement of Fe with Co is just beginning to be studied (for example, PTL 3). On the other hand, a complex oxide Bi(Mg0.5Ti0.5)O3 (hereinafter, also referred to as BMT) is known as a composition that is difficult to synthesize at normal pressure and yields a single phase only at high temperature and high pressure, and this hard-to-prepare composition is also known to be so unstable that the composition, even once prepared, is decomposed at hundreds of ° C. at normal pressure (for example, NPL 3). For this reason, conventional lead-free piezoelectrics have been little studied as to their combination with a BaTiO3 (hereinafter, also referred to as BT) system (see, for example, PTL 4 and NPL 6), but have not been studied as to their application to a BKT-BFO system.
Lead-free piezoelectric elements including a BFO-based piezoelectric composition presumably have a large spontaneous polarization (approximately 100 μC/cm2) (NPL 4) and thus, have been studied actively in recent years. According to the report, however, such piezoelectric compositions having a large spontaneous polarization are difficult to actually obtain on the grounds that, for example, leak current is large and pinning hinders the spontaneous polarization from appearing (NPL 5). Various solutions thereto have been proposed, including, for example, a method involving sintering from very fine starting materials (PTL 1 and NPL 2), a method involving dipping in hot water starting at high temperature, followed by quenching at a very fast rate (NPL 5 and NPL 6), a method involving temperature elevation at a rate as fast as 100° C./second (in order to suppress the evaporation of a highly volatile element such as Bi), followed by sintering in a short time (NPL 7), and a method involving sintering at a temperature near the melting point of a piezoelectric composition to prepare a closely packed sintered body, thereby improving the properties of a lead-free piezoelectric element (PTL 2). Also, a BFO-based lead-free piezoelectric ceramic rich in Co in addition to Fe has been reported (PTL 3).